1. Field of the Invention
The present invention relates to an OFDM receiver and an OFDM signal receiving method for receiving an orthogonal frequency division multiplexing (OFDM) signal and demodulating the OFDM signal.
2. Description of the Related Art
A modulation system called an orthogonal frequency division multiplexing (OFDM) system is used as a modulation and demodulation system of a terrestrial digital broadcasting system. This OFDM system is a system for providing a large number of orthogonal sub-carriers in a transmission band, allocating data to amplitudes and phases of the respective sub-carriers, and digitally modulating a signal according to PSK (Phase Shift Keying) or QAM (Quadrature Amplitude Modulation).
The OFDM system has a characteristic that, since the transmission band is divided by the large number of sub-carriers, although a band per one sub-carrier is narrowed and modulation speed is reduced, transmission speed as a whole is the same as that in the modulation system in the past. The OFDM system also has a characteristic that, since the large number of sub-carriers are transmitted in parallel, symbol speed is reduced. Therefore, in the OFDM system, a time length of a multi-path relative to a time length of a symbol can be reduced and transmission is less susceptible to a multi-path interference. Further, the OFDM system has a characteristic that, since data is allocated to the plural sub-carriers, a transmission and reception circuit can be formed by using, during modulation, an IFFT (Inverse Fast Fourier Transform) arithmetic circuit that performs inverse Fourier transform and using, during demodulation, an FFT (Fast Fourier Transform) arithmetic circuit that performs Fourier transform.
Since the OFDM system has the characteristics described above, the OFDM system is often applied to the terrestrial digital broadcast that is intensely affected by the multi-path interference. As the terrestrial digital broadcast employing such an OFDM system, there are standards such as DVB-T (Digital Video Broadcasting-Terrestrial), ISDB-T (Integrated Services Digital Broadcasting-Terrestrial) and ISDB-TSB (Integrated Services Digital Broadcasting-Terrestrial Sound Broadcasting) (see, for example, “Receiver for Terrestrial Digital Sound Broadcast-Standard (Desirable Specifications) ARIB STD-B30 version 1.1”, Association of Radio Industries and Businesses, decided on May 31, 2001 and revised on Mar. 28, 2002 and “Transmission System for Terrestrial Digital Sound Broadcast ARIB STD-B29 version 1.1”, Association of Radio Industries and Businesses, decided on May 31, 2001 and revised on Mar. 28, 2002).
A transmission signal in the OFDM system is transmitted by a unit of a symbol called an OFDM symbol. This OFDM symbol includes an effective symbol that is a signal period in which IFFT is performed during transmission and a guard interval in which a waveform of a part of the latter half of this effective symbol is directly copied. This guard interval is provided in the former half of the OFDM symbol. In the OFDM system, such a guard interval is provided to improve multi-path resistance. Plural OFDM symbols are collected to form one OFDM transmission frame. For example, in the ISDB-T standard, ten FDM transmission frames are formed by two hundred four OFDM symbols. Insertion positions of pilot signals are set with this unit of OFDM transmission frames as a reference.
In the OFDM system in which the modulation of a QAM system is used as a modulation system for each of the sub-carriers, characteristics of the amplitude and the phase are different for each of the sub-carriers because of the influence of the multi-path and the like during transmission. Therefore, on a reception side, it is necessary to equalize a reception signal to make the amplitude and the phase for each of the sub-carriers equal. In the OFDM system, on a transmission side, pilot signals of a predetermined amplitude and a predetermined phase are discretely inserted in a transmission symbol in a transmission signal. On the reception side, a frequency characteristic of a channel is calculated using the amplitude and the phase of the pilot signals and a reception signal is equalized according to the calculated characteristic of the channel.
The pilot signals used for calculating a channel characteristic are referred to as scattered pilot (SP) signals.